Page 93 - Physics - XII

Q8. In which medium velocity of light is more—water or glass?
Ans. It is more in water because it is having less refractive index (µ = 1.33 for water and for glass, µ = 1.5).

Q9. Defi ne a slab.
Ans. A slab is a piece of transparent material with rectangular faces. All faces are transparent and opposite
faces are parallel.
Q10. What is a prism?
Ans. A prism is a portion of a transparent medium bounded by three plane surfaces inclined to each other
at a certain angle.
Q11. Defi ne angle of prism.
Ans. The angle between the two faces of a prism is called the angle of prism.

Q12. Defi ne lateral displacement.
Ans. The sideway displacement of parallel emergent ray from incident ray is called lateral displacement.
Q13. Why a slab does not deviate and disperse light, whereas a prism does?
Ans. In a slab, refracting faces are parallel. The emergent ray is parallel to the incident ray, hence there
is no deviation and dispersion.
In a prism, the refracting faces are not parallel. The emergent ray is not parallel to incident ray,
hence there is deviation and also there is dispersion.

Q14. What kind of lenses are used in travelling microscope?
Ans. Achromatic converging lenses are used in travelling microscope as objective lens as well as
eyepiece lens.
Q15. What will happen if you go on decreasing the angle of incidence? If you think there is a
minimum, try to fi nd its expression theoretically. What happens when i is less than the
minimum angle of incidence? (NCERT)
Ans. If we go on decreasing the angle of incidence, then a situation arises where the ray will be totally
refl ected at the second refracting surface.

Q16. Does angle of minimum deviation depend upon the colour of the light (wavelength of the light)?
Ans. Yes, since the deviation for diff erent colours are diff erent, so the value of D are also diff erent for
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diff erent colours.

Q17. Why are there two values of angle of incidence for one value of angle of deviation?
Ans. If the emergent ray be reversed, then angle e becomes i and angle i becomes e. The reversed ray
will have same deviation as before. Hence, angle i and e are two diff erent values for same angle of
deviation.

Q18. Interpret the graph between i and D. (NCERT)
Ans. The graph shows the variation of angle of deviation D with the angle of incidence i. As i increases,
the angle of deviation D decreases and reaches a minimum value D and then increases.
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Q19. If the experiment is performed with angle of emergence taken as angle of incidence, will there
be any change in the D values? If yes, why? If not, why not?
Ans. The deviation is defi ned as,
D = i + e – A

So, changing the values of i and e, there will be no change in the D values.

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